The present invention relates to the field of powder metallurgy. More specifically, it relates to the field of preparing amorphous alloys from mixed crystalline elemental metal powders.
In the field of metal alloys it is generally known that amorphous alloys exhibit improvements in various properties when compared with crystalline alloys. These properties include tensile strength, hardness, ductility, corrosion resistance, magnetic properties including hysteresis loss and magnetoelastic effects, and so forth. It is also known that the rate at which an alloy is cooled can be important in determining whether the alloy is amorphous and hence what properties it will have. In general faster cooling can be used to produce amorphous alloys. Specifically, cooling rates on the order of about 10.sup.6 K/s or faster are needed for the preparation of many amorphous alloys. Toward this end various methods have been developed to cool, or "quench," the alloy materials quickly.
One of the most commonly used of the rapid cooling methods is melt-spinning. This is a form of liquid quenching that involves contacting the liquid alloy material with the surface of a thermally conductive material, e.g., a copper surface. This is generally done by laying a liquid coating onto a rapidly spinning wheel. The liquid alloy material cools as it contacts the conductive surface, and the spinning action causes it to form a continuous thin ribbon of solid alloy.
Other methods of liquid cooling include splat quenching, which results in small thin alloy foils, and laser surface modification, such as is disclosed in U.S. Pat. No. 4,613,386, which method is suitable for surface alloying. Other quenching methods include quenching liquid droplets into gas, into liquid, or onto a cool surface, or flame-spraying liquid droplets onto a cool surface. However, all of these quenching methods are generally unsuitable to producing thick amorphous alloy bodies. This is because nucleation and substantial growth of crystalline compounds generally occur due to retardation of the cooling rate, if a thick liquid layer or droplet is being quenched. Thus, the desirable amorphous properties are lost.
Another general method is vapor quenching, which can be performed when a surface alloy coating is desired, such as for the application of corrosion resistant coatings. This type of quenching can be done, for example, by evaporation, a method which tends to result in a fairly poor bond between the coating and the substrate. It requires a relatively long time period and the use of a high vacuum system. A second type of vapor quenching is sputtering. Sputter deposition involves contacting a cold substrate with a plasma containing the desired metal ions. The high energies of the metal ions are used to facilitate the mixing of some of the plasma atoms with the surface atoms. The result is better bonding than that attainable by the evaporation method, but since the procedure must be carried out using an inert gas plasma, a portion of the inert gas is also incorporated in the alloy. Ion implantation techniques can also be used to produce amorphous alloys. For this a high energy ion beam is focused on a crystalline metal surface. The ions penetrate the surface and leave amorphous alloy in their paths.
The above methods are all potentially suited to producing alloys which are amorphous, under the right conditions. These amorphous alloys will in many cases show the improved strength, corrosion resistance, and magnetic properties desired. However, a problem encountered with all of the above described methods is that the alloy being produced, whether as a coating, a ribbon, a foil, or a particle, must be extremely thin. For example, in the case of liquid quenching, the alloy body must generally be less than about 100 microns in thickness in order to enable the cooling rate necessary to ensure an amorphous product. In the case of ion implantation the use of commercially reasonable ion energies results in a thin amorphous layer, i.e., on the order of no more than a few microns, to enable penetration with reasonable ion energies. In the case of evaporation methods the alloy body must be thin to prevent peeling due to inadequate substrate adhesion. Finally, in the case of both evaporation and sputtering the alloy bodies are generally thin because of the extensive time required to build up thicker alloy bodies. Crystallization generally results during processes to compact these ribbons or particles under heat and pressure sufficient to form a monolithic, bulk piece of metal exhibiting bonds between the ribbons or particles whose strength is equivalent to that of the material itself.
An exception to this is disclosed in W. L. Johnson et al., Phys. Rev. Lett. 51 (1983) 415. That publication demonstrates that thin, alternating layers of certain polycrystalline pure metals formed by evaporation or sputtering can be thermally reacted to form an amorphous alloy at temperatures below the selected metals' crystallization temperatures. The alloys formed by this method appear to share two common characteristics: (1) they are formed of metal combinations having a large negative heat of mixing; and (2) the diffusion takes place primarily in one direction, with one metal having very rapid diffusion in the other metal. Again, however, only thin films can be produced, and it is not possible to form complex bulk shapes thereby.
Thus, only alloys produced from powders appear to be suitable for forming bulk shapes. One method of doing this is to ball-mill commercially available coarser elemental metal powders together to mix them, and then to compact and, in some cases, to heat them, at a temperature below the crystallization point, in the desired shape to alloy them. This results in a substantially amorphous alloy body. However, ball-milling has two primary drawbacks: (1) It tends to incorporate significant quantities of impurities into the metal powders; and (2) it is relatively expensive and time-consuming.
In view of the above, there is a need for a method of producing substantially amorphous alloy bodies which are not subject to thickness limitations, do not incorporate significant quantities of impurities, can be densified to theoretical or near-theoretical density in complex bulk shapes, are of substantially uniform composition, and maintain the desirable properties inherent in being amorphous, as discussed above.